Duchenne muscular dystrophy (DMD) is caused by dystrophin deficiency. Gene replacement therapy holds great promise to treat DMD. Adeno-associated virus (AAV) is the leading viral vector for muscle gene therapy. However AAV has a small packaging capacity (~ 5-kb) and it cannot carry the full-length dystrophin cDNA (~12-kb). A microgene is a super-small synthetic dystrophin gene. It contains one-third of the full-length dystrophin coding sequence. Studies from many laboratories suggest that micro-dystrophin can effectively ameliorate muscle disease in dystrophin-deficient mice. Unfortunately, translation to large mammals has so far not been very successful. We recently engineered a new ?R2-15/?R18-19/?R20-23/?C microgene (abbreviated as ?R2 Dys) and tested it adult dystrophin-null dogs by single muscle injection using Y731F AAV-9. Two months later, we observed dramatic reduction of inflammation and fibrosis. Importantly, eccentric contraction-induced damage, a physiological hallmark of DMD, was significantly alleviated. Our study demonstrates for the first time that microdystrophin can treat dystrophinopathy in muscles of large mammals. Our results also suggest that the newly developed Y731F AAV-9 ?R2 Dys vector may hold great translational potential. The overarching goal of this proposal is to address key questions related to future clinical translatio. Specifically, we will pursue two aims including (1) to test the hypothesis that regional intramuscular injection can lead to persistent protection in adult dystrophic dogs. This set of studies will allow us to evaluate the feasibility of life-quality improving therapy in late-stage wheelchair-bound patients in the future; and (2) to test the hypothesis that a single intravenous injection can lead to bodywide amelioration in young dystrophic dogs. DMD affects all muscles in the body. This set of studies will allow us to determine whether systemic intervention can radically change the disease course in affected individuals. Taking together, our studies will break through major barriers in the field and provide the much-needed large animal data to guide human trials in the future.